This study will determine how much of the enhanced stability of rat and rabbit myoglobins could be attributed to an increased saturation of non-polar contacts in the native myoglobin molecule. Scanning microcalorimetry will be used to estimate delta Cp, the change in heat capacity, and delta Hcal, the change in enthalpy, at an alkaline pH for rabbit, rat, armadillo, and pocket gopher myoglobins. The former two myoglobins have been shown to be more stable than sperm whale myoglobin using equilibrium guanidinium chloride denaturation techniques. The hydrophobic stabilization free energy will be computed at 25 degrees C from the change in heat capacity and from current estimates of the respective temperatures at which the entropic and enthalpic hydrophobic contributions to molecular stability are zero. By comparing changes in the observed conformational free energy with changes in the calculated hydrophobic contribution to the unfolding free energy it should be possible to determine what portion of the increased stability of rat and rabbit myoglobins results from an increase in the saturation of nonpolar contacts. By assuming that the interior of myoglobins resembles liquid hydrocarbon, it will be possible to obtain an estimate for the number of additional side chains which must be buried to account for any observed increases in delta Cp. Myoglobins from animal species which may experience temporary anoxia during burrowing or diving may have evolved to resist the denaturing influence of increased hydrogen ion concentrations resulting from glycolytic metabolism during oxygen deprivation. Additional data to test this hypothesis can be gathered from thermal equilibrium unfolding and calorimetric analysis of the myoglobins from armadillo and pocket gophers.